NEC Questions

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Re: NEC Questions

electrofelon:
What happens when the neutral between the service disconnect and utility is compromised?
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Do you think the ground rod will protect against the lost neutral?
It won't! A ground rod does not provide a low enough impedance path to return enough current back to the transformer to provide this protection period! A ground rod that has a impedance of 25 ohms at 120 volts would only allow 4.8 amps to flow back to the transformer. If the service has a unbalance current higher than this you will still have voltage fluctuation and will still damage equipment. This also goes for the thinking that a ground rod will open a breaker. As before only if the breaker has a rating lower than the 4.8 amps ! Most dwellings can have very unbalanced panels because of the load diversities. And many times can have much more current on the neutral than the 4.8 amps a ground rod could provide.

The NEC only requires ground rods for lightning, line surges, unintentional contact with higher-voltage lines, or to stabilize the voltage to earth during normal operation period!!!
Just as it says in 250.4(A)(1):


(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.
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Re: NEC Questions

Interesting thread, a few different topics have sprouted. Yes I agree with you guys for the most part about the ground rods. Actually in the situation I was involved with where the neutral was open, there were no ground rods. The grounding was thru the water pipe (city), which provided a very good return to the source. Their neutral could well have been out for years and might have been for another few years if they didnt have to replace the water pipe. They sure found that something was awry when they cut the old pipe. Anyway in this case the electrode was providing a sufficient path so there was only a few volts of potential between appliances, etc, and ground - essentially normal. I certainly agree that a ground rod will rarely have a low enough resistance to dramaticlly reduce the potential to ground in a situation like this, but it will to some extent. I have made a graph of the voltage to ground vs. resistance of the return path assuming 20A thru a perfect neutral. At zero resistance, there is a potential of zero between the frame of my range and ground. at r=.02, perhaps the normal resistance of the SE neutral and utility messenger, we have less than a volt of potential. An r of 50, maybe two ground rods in favorable soil, would give us a potential of around 100 volts, not much less than 120, but less. feel free to check any of my math and reasoning here ;)
 
Re: NEC Questions

Physis
[/QUOTE] But I don't see their manditory use being enforced through the NEC as being reasonable or intelligent. It seems really fishy to me.
 Please kind sir, do not start me up. Stackers for one thing instead of 3 cables under a staple seems equal in fish aroma as far as I am concerned. So does de rating romex cables run thru joist holes in multiple runs. So does the prohibition of using a larger size romex connector for multiple runs of cables into a panel. How about a six pound limit on light fixtures wall mounted to a switch box . (actually that's about the right weight limit, why can't we just get a switch box that uses 8/32 screws instead of only 6/32 screws?). Why is a little purple wire nut with no -alox squirted inside any better than you squirting a little from your own bottle into a tan twister and using it on alum instead of spending $1.50 each for the purple one. Did you ever get shocked from the old three wire ranges and dryers? Do you know anybody who has? Why are code books so expensive now? Methinks I will probably get into a lot of trouble for this here post.
 
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